Ion implantation is a process used to dope ions into a work piece or target substrate. Large format ion implantation applications, such as implantation into large area flat panels, require extra wide (e.g. 2-6 meter wide) ribbon ion beams. In addition to having to provide high plasma densities necessary for sustaining high ion beam currents, the plasma sources employed in large format applications are targeted to provide very good plasma uniformity (1-2%) over extended lengths. For gaseous precursors, inductively coupled plasma (ICP) sources have proven to be a suitable solution for producing wide ribbon ion beams. For example, an RF inductively coupled plasma source provides reasonably high ion beam currents (about 1 mA/cm Boron) and relatively good uniformity (<2% over 700 mm). However, further extension of the ion beam width is not possible with a single RF antenna due to physical constraints on RF antenna and dielectric RF window lengths. For example, long antennas have high antenna inductance, which, for usual RF frequencies, requires matching units with non-physical tuning and loading capacitors values.
Moreover, overall antenna length is limited because an antenna may be designed to not exceed ¼ of the RF electromagnetic wavelength due to the standing wave effect that might develop in the antenna loops. Due to this consideration, for an operating frequency of 13.56 MHz, the maximum antenna length (taken by summing up the lengths of all antenna turns) is about 5 meters.
In addition, dielectric windows, which allow RF power transmission therethrough and provide vacuum sealing of plasma chambers, are constrained in size because windows generally must be made thicker as they are made larger. For example, in order to sustain 1 atmosphere of pressure under 250-300° Celsius thermal stress, a 1 meter long and 15 centimeter high window must have a thickness of about 15 millimeters in the case of quartz and about 10 millimeters in the case of alumina. However, thick windows provide poor RF power coupling with a detrimental effect on plasma density.